Condensible gas cooling system

ABSTRACT

A workpiece cooling system and method are disclosed. Transferring heat away from a workpiece, such as a semiconductor wafer during ion implantation, is essential. Typically this heat is transferred to the workpiece support, or platen. In one embodiment, the desired operating temperature is determined. Based on this, a gas having a vapor pressure within a desired range, such as 10-50 torr, is selected. This range is required to be sufficiently low so as to be less than the clamping force. This condensible gas is used to fill the volume between the workpiece and the workpiece support. Heat transfer occurs based on adsorption and desorption, thereby offering improved transfer properties than traditionally employed gases, such as helium, hydrogen, nitrogen, argon and air.

BACKGROUND OF THE INVENTION

Ion implanters are commonly used in the production of semiconductorwafers. An ion source is used to create an ion beam, which is thendirected toward the wafer. As the ions strike the wafer, they dope aparticular region of the wafer. The configuration of doped regionsdefines their functionality, and through the use of conductiveinterconnects, these wafers can be transformed into complex circuits.

A block diagram of a representative ion implanter 100 is shown inFIG. 1. An ion source 110 generates ions of a desired species. In someembodiments, these species are atomic ions, which may be best suited forhigh implant energies. In other embodiments, these species are molecularions, which may be better suited for low implant energies. These ionsare formed into a beam, which then passes through a source filter 120.The source filter is preferably located near the ion source. The ionswithin the beam are accelerated/decelerated in column 130 to the desiredenergy level. A mass analyzer magnet 140, having an aperture 145, isused to remove unwanted components from the ion beam, resulting in anion beam 150 having the desired energy and mass characteristics passingthrough resolving aperture 145.

In certain embodiments, the ion beam 150 is a spot beam. In thisscenario, the ion beam passes through a scanner 160, which can be eitheran electrostatic or magnetic scanner, which deflects the ion beam 150 toproduce a scanned beam 155-157. In certain embodiments, the scanner 160comprises separated scan plates in communication with a scan generator.The scan generator creates a scan voltage waveform, such as a sine,sawtooth or triangle waveform having amplitude and frequency components,which is applied to the scan plates. In a preferred embodiment, thescanning waveform is typically very close to being a triangle wave(constant slope), so as to leave the scanned beam at every position fornearly the same amount of time. Deviations from the triangle are used tomake the beam uniform. The resultant electric field causes the ion beamto diverge as shown in FIG. 1.

In an alternate embodiment, the ion beam 150 is a ribbon beam. In suchan embodiment, there is no need for a scanner, so the ribbon beam isalready properly shaped.

An angle corrector 170 is adapted to deflect the divergent ion beamlets155-157 into a set of beamlets having substantially paralleltrajectories. Preferably, the angle corrector 170 comprises a magnetcoil and magnetic pole pieces that are spaced apart to form a gap,through which the ion beamlets pass. The coil is energized so as tocreate a magnetic field within the gap, which deflects the ion beamletsin accordance with the strength and direction of the applied magneticfield. The magnetic field is adjusted by varying the current through themagnet coil. Alternatively, other structures, such as parallelizinglenses, can also be utilized to perform this function.

Following the angle corrector 170, the scanned beam is targeted towardthe workpiece 175. The workpiece is attached to a workpiece support. Theworkpiece support provides a variety of degrees of movement.

The workpiece support is used to both hold the wafer in position, and toorient the wafer so as to be properly implanted by the ion beam. Toeffectively hold the wafer in place, most workpiece supports typicallyuse a circular surface on which the workpiece rests, known as a platen.Often, the platen uses electrostatic force to hold the workpiece inposition. By creating a strong electrostatic force on the platen, alsoknown as the electrostatic chuck, the workpiece or wafer can be held inplace without any mechanical fastening devices. This minimizescontamination and also improves cycle time, since the wafer does notneed to be unfastened after it has been implanted. These chuckstypically use one of two types of force to hold the wafer in place:coulombic or Johnson-Rahbeck force.

The workpiece support typically is capable of moving the workpiece inone or more directions. For example, in ion implantation, the ion beamis typically a scanned or ribbon beam, having a width much greater thanits height. Assume that the width of the beam is defined as the x axis,the height of the beam is defined as the y axis, and the path of travelof the beam is defined as the z axis. The width of the beam is typicallywider than the workpiece, such that the workpiece does not have to bemoved in the x direction. However, it is common to move the workpiecealong the y axis to expose the entire workpiece to the beam.

Another important function of the workpiece support is to provide a heatsink for the workpiece. For example, during ion implantation, asignificant amount of energy, in the form of heat, is imparted to theworkpiece. This heat, left unregulated, could affect the properties ofthe workpiece that is being implanted. Therefore, this heat ispreferably transferred away from the workpiece and to the workpiecesupport. The workpiece support then dissipates the heat. In certainembodiments, fluid is passed through conduits within the workpiecesupport, which allows the heat to be transferred to the fluid and awayfrom the workpiece support. Other methods of cooling the workpiecesupport are also well known in the art.

In certain embodiments, the heat is transferred from the workpiece tothe workpiece support, simply through the physical contact between thetwo components. However, tests have shown that even though the workpieceand the support appear to be in physical contact; at a microscopiclevel, there is relatively little actual contact between the twocomponents, due to imperfections and the roughness of the adjacentsurfaces.

The ion implantation system described above is preferably housed in anenvironment that is near vacuum conditions. In fact, the pressure withinthis environment is typically less than 10⁻⁵ Torr. Since the surroundingenvironment is nearly an absolute vacuum, there are no other mediumsthrough which the heat can be transferred. Consequently, heat transferis far less efficient than desired.

One technique to improve the transfer of heat from the workpiece to thesupport is the use of “back side gas”. FIG. 2 shows a simplifiedillustration of this technique. Briefly, the workpiece 200 is clamped tothe support, using mechanical or electrostatic means. Then, conduits 220in the workpiece support 210 transfer gas 250 to the volume between theworkpiece 200 and the support 210, also known as the wafer/plateninterface.

A simple figure showing this heat transfer mechanism is presented inFIG. 3. Heat transfer occurs when a gas molecule collides with theworkpiece 200, absorbing heat from the workpiece 200. This gas moleculelater collides with the workpiece support 210, imparting the transferredheat to the support. The workpiece support serves as a heat sink andmaintains an acceptable temperature. In some embodiments, the workpiecesupport is cooled by passing fluid through internal cooling conduits230. The flow of back side gas may be controlled by a mass flowcontroller 250.

Since it is these gas molecules that transfer the heat, an increase inthe number of molecules, such as by an increase in pressure, results inimproved heat transfer. However, the pressure of the back side gas hasan upper limit; as the pressure of the back side gas increases, itbegins to overcome the clamping forces, thereby pushing the workpieceaway from the support. This reduces the actual physical contact betweenthe two surfaces and significantly lowers the heat transfer. Thisreduction occurs at very low pressure, such as less than 50 Torr in anion implantation environment. Excessive pressure may also cause damageto the workpiece. Furthermore, a large increase in the number ofmolecules serves to increase collisions between molecules, and thereforereduces the heat transferred between the solids.

As described above, back side gas aids in the heat transfer as gasmolecules receive heat from the workpiece and transfer that heat to theworkpiece support. As is well known, at gas-solid interfaces, there isan efficiency at which this heat transfer occurs, which is dependent onboth the type of gas molecule and the type of solid. This efficiency isdescribed by the accommodation coefficient, which has a value between 0(no heat transfer) and 1 (perfect heat transfer). The accommodationcoefficient (α) is typically defined as:

α=(T _(r) −T _(i))/(T _(s) −T _(i)),

-   -   where T_(r) is the temperature of the reflected molecules (i.e.        gas molecules after they reflect off the solid surface);    -   T_(i) is the temperature of the incident molecules (i.e. gas        molecules before they hit the solid surface; and    -   T_(s) is the temperature of the solid surface.

Lighter gases, such as helium and hydrogen, typically have loweraccommodation coefficients then heavier gasses, such as nitrogen, argonand air. In addition, the solid surface contributes to the accommodationcoefficient, as some solids provide better heat transfer than others.Returning to FIG. 3, assume that the accommodation coefficient betweenthe gas molecule and the workpiece 200 is α₁ and the accommodationcoefficient between the gas and the workpiece support 210 is α₂. Asmolecules collide with the workpiece, these molecules absorb heat fromthe workpiece, in proportion to the accommodation coefficient α₁. Thesemolecules later collide with the support 210, imparting their heat inproportion to the accommodation coefficient α₂. Thus, the actual heattransfer between the workpiece and the workpiece support is proportionalto α₁*α₂. For example, if the accommodation coefficient at one surfacewith a specific gas is 0.9 and the coefficient at the other surface withthat gas is 0.7, then the heat transfer between the two surfaces is only63% efficient. Heavier gases can increase these coefficients, however,lighter gas molecules move more rapidly and thus transfer heat morequickly. This may favor their use over heavier species, despite thedifference in accommodation coefficient.

In many environments, it is important to keep the workpiece within apredetermined temperature range. Thus, efficient heat transfer from theworkpiece to the workpiece support is essential. Therefore, it would bebeneficial to develop a system and method for improving the cooling ofworkpieces, especially semiconductor wafers in an ion implantationsystem.

SUMMARY OF THE INVENTION

The problems of the prior art are overcome by the workpiece coolingsystem and method described in the present disclosure. Typically thisheat is transferred to the workpiece support, or platen. In oneembodiment, the desired operating temperature is determined. Based onthis, a gas having a vapor pressure within a desired range, such as10-50 torr, is selected. This range is required to be sufficiently lowso as to be less than the clamping force. This condensible gas is usedto fill the volume between the workpiece and the workpiece support. Heattransfer occurs based on adsorption and desorption, thereby offeringimproved transfer properties than traditionally employed gases, such ashelium, hydrogen, nitrogen, argon and air.

BRIEF DESCRIPTION OF FIGURES

FIG. 1 represents a traditional ion implanter;

FIG. 2 represents a cross-section of a workpiece and support accordingto one embodiment;

FIG. 3 represents a simplified illustration showing the heat transfermechanisms of the prior art;

FIG. 4 represents a simplified illustration showing the heat transfermechanism described the present disclosure; and

FIG. 5 represents a flow chart illustrating process steps used inaccordance with one embodiment.

DETAILED DESCRIPTION OF THE INVENTION

As described above, maintaining the temperature of a workpiece, such asa semiconductor wafer in an ion implantation process, is essential.Current techniques for maintaining the temperature of a workpiece relyon the transfer of heat from the workpiece to the workpiece support,such as the platen, which is physically contacting the workpiece. Someembodiments augment this heat transfer mechanism by delivering “backside gas” in the volume between the workpiece and the support. These gasmolecules act to transfer the heat (or a portion of it) from theworkpiece to the support. However, as described above, this heattransfer mechanism is not as efficient as desired.

Returning to FIG. 2, a cross section of a workpiece support 210, with anattached workpiece 200 is shown. The workpiece support may have twotypes of conduits. Conduit 220 brings gas 250 to the back side of theworkpiece, in the volume between the workpiece and the support. The gas250 is stored preferably in a central reservoir, such as a tank, and maypass through a mass flow controller or pressure regulator 240 toregulate its flow through conduit 220. In certain embodiments, the smalltrench 260 is provided in the upper surface of the support 210 to allowan unobstructed path for the gas 250 to enter the volume. The MFC orpressure regulator 240 controls the flow of gas to achieve the desiredgas pressure. As noted above, the pressure is preferably carefullycontrolled, as excessive pressure may serve to lift the workpiece awayfrom the support, or may damage the workpiece.

In some embodiments, a second conduit 230 is used to circulate a fluidwhich is used to cool the workpiece support. For example, water, air ora suitable coolant may be circulated through the internal conduit 230 ofthe workpiece so as to conduct heat away from the platen.

Each ion implantation process has a preferred operating temperaturerange. For example, many ion implantations are performed within atemperature range of 0° C. to 50° C., and more commonly at roomtemperature (15-30° C.). Others are performed at cryogenic temperatures,such as less than −50° C. Other implants are performed at elevatedtemperatures, such as greater than 100° C. Once the desired operatingrange is determined, an appropriate gas is selected. The gas should beone that has a sufficiently low vapor pressure at the desired operatingtemperature. For example, at room temperature, water has a vaporpressure of about 20 Torr. For a cryogenic implant at −100° C., propanehas a similar vapor pressure. Ammonia (NH₃) would also be suitable forlow temperature implants. Its vapor pressure at −80° C. is approximately30 Torr. For higher temperature implants, substances such as glycerinecan be used, as its vapor pressure is approximately 40 Torr at 200° C.

As stated above, the vapor pressure of the gas in the working regionmust be less than the clamping force exerted on the workpiece, such thatthe workpiece remains undamaged and in contact with the workpiecesupport. In other words, the pressure exerted by the gas, multiplied bythe area of the workpiece, determines the force being exerted on theworkpiece in the direction away from the workpiece support. Inopposition to this force is the clamping force. In order for theworkpiece to remain in contact with the support, the clamping force mustbe greater than the gas pressure, multiplied by the area of theworkpiece. Since the area of the workpiece is fixed, the gas pressuremust be controlled to insure that this condition is satisfied.

In many embodiments, the desired vapor pressure is between 1 and 50Torr, although other ranges are possible and within the scope of thedisclosure. The selected gas is delivered through conduit 220. Forexample, as noted above, at room temperature, water has a vapor pressurebetween 10 and 20 Torr. For an ion implantation occurring at roomtemperature, water vapor is delivered to the volume between theworkpiece and the support. This can be done using the conduit 220 shownin FIG. 2. The water vapor is pressurized using MFC or pressureregulator 240 such that the vapor phase and liquid phase are inequilibrium. When this happens, a thin film 205 of water vapor adsorbson the back surface of the wafer 200. A thin film 215 also adsorbs onthe top surface of workpiece support 210. By creating a film of gasvapor on each surface, the heat transfer mechanism is changed.

FIG. 4 shows a simplified representation of the heat transfer mechanism.In this scenario, gas vapor molecules adsorb to the film 205 on thesurface of the workpiece. A different water vapor molecule, already atthe elevated temperature, is displaced and desorbs from the film 205.This displaced molecule is then adsorbed into the film 215 on the topsurface of the support 210. Again, a different molecule is thendisplaced, which is at the reduced temperature of the support. Since themolecule being desorbed is at or nearly at the temperature of the solid,(i.e. T_(r) is roughly equal to T_(s)), an accommodation coefficient ofnearly 1 can be realized.

FIG. 5 represents a flowchart of the process steps previously described.As stated above, first a desired operating temperature is determined, asshown in Box 400. Then, a suitable gas is selected, based on thisoperating temperature, as shown in Box 410. Again, the vapor pressure ofthis gas at the desired temperature is preferably sufficiently low so asnot to damage the workpiece or overcome the clamping force. As notedabove, if necessary, the MFC or a pressure regulator 240 can be used toreduce the working pressure below the vapor pressure of the workingfluid. The selected gas is then delivered into the volume between theworkpiece and the support, as shown in Box 420. Preferably, sufficienttime is allowed to permit the gas to reach steady-state conditions inthis volume, as shown in Box 430. Steady-state conditions are met whenthe gas pressure is equal to the vapor pressure. This allows the gas toadsorb on the back side of the workpiece and the top surface of thesupport. Once steady-state conditions have been reached, the ionimplantation process can begin, as shown in Box 440.

As shown in Box 430, it is preferably to allow the vapor to reachsteady-state conditions before beginning the ion implantation process.This can be accomplished in a number of ways. In one embodiment, theprocess cycle time is slowed to allow steady-state conditions to bereached. In other words, once a new workpiece or wafer has been placedon the platen, the flow of vapor is started. Ample time elapsed beforethe ion implantation process begins. This time permits the vaporpressure and adsorbed film to reach a steady state value. This method isstraightforward, but may impact throughput, depending on the timerequired for equilibrium to be reached.

Other methods can be used to reduce the amount of time required for thevapor to reach steady-state conditions. For example, the adsorbed vaporfilm on the workpiece support may be maintained during wafer exchange byreducing the temperature of the support. The colder temperature willliquefy or perhaps freeze the film. Alternatively, the vapor may beintroduced through a porous medium that is part of the workpiecesupport. Finally, coating the workpiece with the selected gas, liquid,or material before it is placed on the support can reduce the requiredtime. For example, a workpiece may be exposed to water vapor prior tobeing placed on the workpiece support, and then chilled to retain thewater until placed on the support. In one embodiment, the wafer orientstation is used to simultaneously apply the water vapor and chill thewafer (during the orient). After this is completed, the wafer will beplaced on the workpiece support and a steady state vapor pressureestablished as the wafer and support temperature equalizes.

While the disclosure describes ion implantation, the disclosure is notlimited to this embodiment. The method and system described herein canbe used in any application utilizing a workpiece and a workpiecesupport, especially in a vacuum environment.

1. A method for transferring heat from workpiece while said workpiece isbeing processed, said workpiece mounted on a workpiece support,comprising: a. Determining an operating temperature range for saidprocessing; b. Selecting a gas, said gas having a vapor pressure withina desired range at said operating temperature range; c. Delivering saidgas in the volume between the back side of said workpiece and the topsurface of said workpiece support; and d. Processing said workpiece. 2.The method of claim 1, further comprising the step of waiting for saidgas to reach equilibrium in said volume before said workpiece isprocessed.
 3. The method of claim 2, wherein a film of liquid isproduced on said back side of said workpiece and said top surface ofsaid workpiece support.
 4. The method of claim 1, wherein a force isapplied to hold said workpiece on said workpiece support, and saiddesired range of said vapor pressure produces an opposing force that isless than said force holding said workpiece.
 5. The method of claim 1,wherein said gas is delivered at a pressure equal to said vaporpressure.
 6. The method of claim 1, further comprising the step ofcooling said support before said processed workpiece is removed.
 7. Themethod of claim 1, wherein said operating temperature range is between 0and 50° C., and said selected gas comprises water vapor.
 8. The methodof claim 7, wherein said vapor pressure is between 10 and 50 torr. 9.The method of claim 1, wherein said operating temperature range is lessthan −50° C., and said selected gas comprises ammonia.
 10. The method ofclaim 1, wherein said operating temperature range is greater than 100°C., and said selected gas comprises glycerin.
 11. The method of claim 1,wherein said process comprises ion implantation.
 12. A system fortransferring heat away from a workpiece, while said workpiece is beingprocessed at a predetermined operating temperature range, comprising: a.a workpiece support upon which said workpiece is placed, such that thetop surface of said support contacts the back side of said workpiece; b.means for holding said workpiece on said workpiece support, said meansexerting a force on said workpiece; c. a conduit for providing gas tothe volume defined by the back side of said workpiece and said topsurface of said workpiece support; d. and a reservoir for holding saidgas, wherein said gas has a vapor pressure at said operating temperaturerange, wherein said vapor pressure which produces an opposing force onthe workpiece that is lower than said force exerted by said means tohold said workpiece.
 13. The system of claim 12, wherein said operatingtemperature range is between 0 and 50° C., and said gas comprises watervapor.
 14. The system of claim 12, wherein said operating temperaturerange is less than −50° C., and said gas comprises ammonia.
 15. Thesystem of claim 12, wherein said operating temperature range is greaterthan 100° C., and said gas comprises glycerin.
 16. The system of claim12, wherein said conduit is located within said workpiece support, andsaid gas passes through said workpiece support to reach said volume. 17.The system of claim 12, further comprising a mass flow controller orpressure regulator located between said reservoir and said volume. 18.The system of claim 17, wherein said mass flow controller delivers saidgas at a pressure equal to said vapor pressure.